Adaptive filters, sometimes called dynamic filters or auto-adaptive filters have a transmission frequency band which is shifted in response to the content of the applied signal. They are mainly used for noise reduction or signal restoration in audio frequency transmission systems. The book by R. Masschu entitled Techniques du Magnetophone Editions Radio, Paris 1974, describes on page 242 et seq., the principals of operation of several of these systems.
These prior art systems generally function either by rejection of a portion of the frequency spectrum containing little useful information or by expansion of the entire signal followed by compression of the low level signal components. The first method is a one way process and is employed in the well known DNE and DNF systems and the latter method, which is a complimentary function process, is used in the well known Dolby A and B systems and in the ANRS system. Moreover, the portion of the spectrum containing the desired information may be fixed as in the DNL and Dolby A systems or it may be varied in relation to the content of the desired part of the spectrum as in the Dolby B, ANRS and DNF systems.
These one way noise reduction systems are designed for incorporation in sound transmission or sound reproduction apparatus to attenuate the noise components which are present in the rejected part of the signal spectrum. They are applicable to any signals whatever and are therefore compatible with all types of systems because their use only affects the audible noise level of the system in which they are used.
The complimentary function systems, which are designed to reduce the effect of the noise introduced into the signal during processing of the signal, carry out a portion of the process before transmission or recording. Thereafter, and prior to reproduction the signal is restored to its non-distorted condition by a reverse or complimentary process. These types of systems are not, therefore, compatible.
These types of systems have in common the fact that they include means fulfilling the functions of a frequency meter to detect the presence of a signal within the operating frequency bands. Such means vary the cut-off frequencies of the associated band pass filters in relation to the bandwidth of the incoming signal. Inasmuch as this frequency meter function can be independent, at least to some degree, of the signal levels, the desired adjustment of the cut-off frequencies of the associate filters is only approximately obtained, for example, by means of rectifiers, limiters and fixed-frequency filter circuits.
The possible uses of these systems are thus limited because of this defect which results from the uncertainty about the positioning of the cut-off frequencies of the filters relative to the useful part of the signal spectrum. Consequently, these prior art systems must either accept a reduction of the transmitted spectrum, which thus downgrades the quality of the reproduced signal, or reverse a safety margin which also reduces their efficiency. Moreover, the complimentary function systems require an accurate level adjustment of the signals at the moment of their restoration or reproduction.
The performance of audio reproducing apparatus could be significantly improved if there were available a sure means of determining, as a function of precise criteria, the useful bandwidth of a signal. In other respects, such a means would enable new ways of resolving many technical problems, particularly all those where a fixed bandwidth is assigned for transmission of a signal when in fact the bandwidth effectively used is, at least instantaneously, much narrower, thus resulting in an efficient use of the transmission means.